The present invention relates to precision forming of sheet metal and more particularly to a simplified and lower cost method for stamping raised relief features in sheet metal without inducing significant metal stresses that warp the sheet. A particular application is formation of ridges that help guide paper along high speed paper paths formed of sheet metal.
Pressing or stamping of raised relief features in sheet metal is a common operation used in fabricating many industrial components. In such processes, the sheet metal is stressed as the feature is drawn, or stretched, away from the initial planeness of the base sheet of metal. Such stresses under prior art processes warp the base of sheet metal from which the feature is drawn.
In applications where a high degree of planeness in the base sheet of a sheet metal component is required after the drawing process, prior art methods and procedures have countered the warping of the base sheet through a number of techniques. In one prior art technique, planeness can be improved by stretching the work piece, especially when the stretched sheet is rolled while stretched. U.S. Pat. No. 6,216,521 describes one such process in relation to production processes for hot rolled and quenched metal sheets. Among the shortcomings of this techniques are the requirement for expensive high pressure hydraulic equipment and gripping fixtures as well as indentations or distortions introduced into the part by the gripping fixtures themselves. In another prior art technique to increase planeness, a component is heated to soften the metal before the raised relief feature is drawn. A shortcoming of this second method is that the subsequent cooling process itself may introduce warping in the base sheet. In addition, temperatures that are hot enough to soften the metal may adversely alter the crystalline characteristics of the metal. Yet another prior art technique involves striking, or pushing small xe2x80x9cdimplesxe2x80x9d into the workpiece to introduce surface stresses that offset the stresses previously introduced by the drawing process. Such striking process often requires manual manipulation since variations in the workpiece base substrates make the stresses introduced by drawing irregular. Such manual manipulation takes time, is imprecise, depends greatly upon the intuition and skill of the manipulator, and other ways significantly increases costs while diminishing quality.
One example of a component with raised relief features that requires a high degree of planeness in the base sheet is a substrate guide in a high speed electrostatographic printer. This guide is designed to help position any number of printing cut sheet or web substrates, including paper, transparencies, cut sheets, other plastics and, generally, any planar material suitable for printing. An example of such a paper guide component is shown in FIG. 1. Such a guide component 10 is typically used in the portion of a printer or copier that guides the substrate to the photoreceptor. Raised relief ribs 14-20 and similar ribs are designed to reduce friction as paper slides over guide component 10 as well as to help paper continue in a straight path from paper feed system to the photoreceptor/substrate image transfer area, while inhibiting skew of the paper. Additionally, ribs such as 14-20 reduce the area of contact between guide member 10 and the copy substrate, thereby minimizing the risk that contaminant toner that falls onto guide component 10 will smudge the reverse side of the copy substrate. Ribs 14-16 are skewed in relation to the paper path direction in order to prevent paper edge jamming on rib edges and also to guide paper sizes from A5 to A4, in various registration modes.
FIGS. 2-4 provide close-up and cross-sectional views of guide component 10. In FIG. 2, a series of ribs are shown in detail with cross-sectional perspectives indicated by lines Axe2x80x94A and Bxe2x80x94B. FIG. 3 shows the elevated cross-sectional view along line Axe2x80x94A of FIG. 2. In this view, two ribs, 15 and 16 are in raised relief from base sheet 12. FIG. 4 shows the elevated cross-sectional view along line Bxe2x80x94B of FIG. 2. This cross-sectional view shows the cross-sectional raised relief profile of rib 15 in relation to base sheet 12. In the embodiment shown in FIGS. 1-4, ribs 14-17 in guide component 10 are in the range of 15 mm long and are drawn in a relief of approximately 1.2 mm above base plate 12. Ribs 14-17 are drawn with a high aspect ratio as shown particularly in FIG. 3. Of course, the shape and dimensions of paper guide components such as guide component 10 with ribs 14-20 vary greatly depending upon the particular apparatus and function that they are to serve.
Under prior art processes, guide component 10 is manufactured in a progressive die cut process as indicated in FIG. 5. This process is conventionally finished with a manual striking process to straighten the part after the progressive cutting and stamping procedures. Moving from right-to-left in FIG. 5, the die cut process begins at step 1 with a blank sheet 50 of stainless steel comprised of 304 alloy or similar material. Piercing slots are first begun in step 2 in a die-cut along the edges leading to a bending and cutting step in step 3 that forms slotted mounting features such as lugs 51 along each side. Another set of piercing cuts are made at step 3 to begin separation of the various guide components 10 from each other. A last separation cut is made at step 4 to separate each guide component 10 except for its end region proximate to mounting lugs 51.
At step 6, ribs such as ribs 14-20 are drawn in a stamping process. In FIG. 5, this drawing process takes several intermediate iterations in order to minimize the stamping force required any one step of the drawing process. At step 7, a striking process is applied by stamping strategically placed dimples into guide component 10 in an attempt to offset the internal stresses causes by drawing step 6. At step 8, the long edges are folded over to increase the end-to end rigidity of guide component 10 and to form crested flat region 21, shown in FIG. 2. At the far left of FIG. 5, mounting lugs 51 are finally free cut to fully separate each guide component 10 from the base sheet 50, thereby to form the completed guide component.
One additional step is generally after the free cut is made. Although the striking step of step 7 attempts to remove stress and to thereby provide straightness along the long dimension of guide component 10, such automated striking process rarely succeeds in obtaining the desired straightness. Accordingly, an additional manual re-striking step is required to obtain the required flatness. This re-striking process adds cost, complexity, and, because it is manually performed, imprecision to the finished guide component 10. The particular purpose of this re-striking process is to remove the stresses introduced along the long dimension of guide component 10 by the drawing process at step 6 that forms the ribs.
Prior art achievement of flatness across the long dimension of various portions of guide component 10 has been difficult. Without hand straightening, typical flatness of a formed guide component 10 under the prior art was as follows:
(A) end-to-end flatness over ribs such as ribs 14-20: xe2x89xa63.0 mm;
(B) end-to end flatness of ribbed base sheet 12: xe2x89xa63.0 mm; and
(C) end-to-end flatness over crested flat region 21: xe2x89xa63.0 mm.
The reason for such lack of flatness is that the drawing process stretches metal in the vicinity of the drawn features, thereby creating differential stresses in various regions of the base metal. The result is that drawing the raised relief ribs such as 14-20 not only causes ripples in the ribbed base sheet 12 but, in addition, causes the metal to ripple in the crested flat region 21 as well. With manual straightening using an expensive and tedious striking process, flatness can be improved to the following acceptable levels:
(A) end-to-end flatness over ribs such as ribs 14-20: xe2x89xa60.4 mm;
(B) end-to end flatness of ribbed base sheet 12: xe2x89xa60.5 mm; and
(C) end-to-end flatness over crested flat region 21: xe2x89xa60.5-0.75 mm.
For a typical guide component 10 of approximately 320 mm, this means that end-to-end flatness of the critical crested flat region 21 must minimally be maintained within 0.2% (0.751320%). Significant improvements in flatness would be greatly desired. Referring to FIG. 6, the reason that such flatness is important is that guide component 10 acts as an intermediate between the paper feed system and the image transfer stage by guiding the copy substrate to the exact image transfer area at the photoreceptor. Crested flat region 21 of guide component 10 sets the gap through the copy substrate must move, thereby determining the substrate proximity to the photoreceptor. The gap, in turn, is set by fastening lug mounts 51 at the extreme ends of guide component 10. If the 0.2% end-to-end flatness of crested flat region 21 of guide component 10 is not met, then at least portions of the image transfer gap will be either too large or too small.
The particular dimension of the gap is optimized for the substrate size range specified in the machine performance specification. In a typical specification for an electrostatographic printer, the spacing of guide component 10 to the photoreceptor is set at only 0.7xc2x10.2 mm. As discussed above, in order to achieve such flatness, an extra striking step is included in the progressive die press operation of the prior art. This re-striking step introduces extra surface stresses that offset the warping induced by the rib forming. As an extra and as a manual process, this extra straightening process adds the cost of an additional process, including set up, execution, and testing time. Also, additional striking machinery and tooling is required, both of which must be maintained.
As additional background, many prior art processes for stamped and drawn parts provide for cuts and slits to be formed within the base sheet of a part. For instance, sheet metal parts formed into housings for machinery producing heat such as air conditioners have die cut slits that are then bent during a drawing process to form vents. Double slits around raised rib-like parts are also known in sheet metal used as lattice support during construction of plaster-faced walls. In none of these known applications, however, is precision flatness a requirement of the final component.
In sum, considerable advantages over prior art manufacturing techniques for components such as guide component 10 would be realized if any extra straightening process, especially a manual process, could be eliminated and if a simplified and less expensive manufacturing process yielded components with even greater straightness than parts made with prior art processes.
One aspect of the invention is a guide member for guiding a substrate along a paper path, said guide member comprising: (a) a ribbed base sheet having a long dimension and a plurality of end regions; (b) a substrate guide rib formed in raised relief from the ribbed base sheet, said rib having two long and two short sides, each side having a base region; (c) a cut formed along one long side of the rib proximate to the base region of such long rib side; (d) a crested flat region formed parallel to the ribbed base sheet along its long dimension; and (e) mounting fixtures located proximate to the end regions.
Another aspect of the invention is an electrostatographic marking system, comprising a substrate guide member comprising: a ribbed base sheet having a long dimension and a plurality of end regions; a plurality of substrate guide ribs each formed in raised relief from the ribbed base sheet and having two long and two short sides with each side having a base region; a cut formed along at least one long side of each rib proximate to the base region of such long rib side; a crested flat region formed parallel to the ribbed base sheet along its long dimension; and mounting fixtures located proximate to the end regions.
Yet another aspect of the invention is a process for forming a substrate guide member, comprising: forming the flattened outline of the substrate guide member out of sheet metal; cutting a slit in the sheet metal at the location that will become the base of at least one long side of a raised relief rib formed on the substrate guide member; drawing the raised relief rib at the location such that the cut slit is proximate the base of a long side of a raised relief rib; and bending one long edge of the flattened outline to form a crested flat region of the substrate guide member.